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Erratic Boulders - Tracking The Glaciers

Most of Whidbey Island's terrain is relatively smooth because it has been sculpted in glacial till and sediments. However,  there is occasionally a large boulder perched on the ground surface. These are erratic boulders, which were derived from the basement rocks of northern Washington and southern Canada. The rock types of these boulders can be traced back to possible sources and give clues to directions of ice movement.

The Fort Casey Erratic


Access: From Coupeville, drive south along Engle Road for 4.1 miles to Fort Casey State Park. Turn right into the state park and go up the hill for about a quarter of a mile to a day use area on your right. You may need to park farther up by the park offices. (be sure to display your State Park pass). 


About 100 yards NNW, behind a restroom, is a large greenstone boulder.  Again, the source was likely outcrops on Fidalgo Island.

 Fort Casey Erratic

The Coupeville Erratic


Access:  From Highway 20 at the stoplight, turn right (south) onto South Main Street. About 0.1 mile south, the erratic sits behind a coffee shop and in front of the Big Rock Apartments.


The 22-foot-high erratic is covered with English ivy, making the rock itself difficult to see. You wouldn't be able to look anyway because it is on private land. It consists of greenstone, similar to outcrops ranging from the Deception Pass Bridge to Mount Erie (24 miles north of here). It, too, was dropped here during the last glaciation.

  Coupeville Erratic

The Clinton Erratic


Access: After arriving on the ferry at the Clinton Ferry Terminal, travel inland on Highway 525. O.2 miles from the terminal, turn left onto Humphrey Road; After 0.4 miles, turn left onto Berg Road; After 0.5 Miles, turn right onto Conrad Street; after 0.6 miles, stop near the large greenstone boulder near the drainage ditch.


This boulder was deposited during the most recent glacial advance into Puget Sound. The greenstone was likely derived from greenstones that make up much of Fidalgo Island, 35 miles north-northwest of Clinton.


The Waterman Erratic


Access: From Langley's downtown, go west on Second Street, which becomes Saratoga Road. 2.5 miles from Langley is the Saratoga Woods Preserve, on the west side of the road. From the Preserve parking lot, follow trails west or northwest. Trails are named but the signage is mostly lacking. You will reach an old, overgrown landing strip. turn north and follow the landing strip to the Bent Tree Trail. On the western (left) side, go into a shallow depression, which contains the erratic.


The erratic is a big guy—38 feet high and 60 feet long. It is believed to be the second largest glacial erratic in Washington. It is a foliated greenschist and likely was broken from outcrops on Fidalgo Island about 30 miles northwest of here during the last glaciation.

 Waterman Erratic

The Erratic at Double Bluff


Access: About 1 mile east of the stoplight in Freeland is the intersection with Double Bluff Road. Turn right onto Double Bluff Road and go for about 2.0 miles south to the parking lot for Double Bluff Beach. If you walk along the beach, do it at low tide!  At about 0.6 miles SSW from the parking lot is a 10-foot-diameter erratic fossiliferous sandstone boulder. Brought to this point during the last glaciation, 15,000 to 11,000 years ago, the source is believed to be from outcrops south of Sedro-Wooley (28 miles north of here).

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